Add compatibility with `go install`

[sodul]

I tried to get buildx through go install and even the gobin (now deprecated) utility but could not get it to work.

This is with go 1.16.3 on macOS Intel.

> go install github.com/docker/buildx/cmd/buildx@v0.5.1
go install github.com/docker/buildx/cmd/buildx@v0.5.1: github.com/docker/buildx@v0.5.1
	The go.mod file for the module providing named packages contains one or
	more replace directives. It must not contain directives that would cause
	it to be interpreted differently than if it were the main module.

> gobin github.com/docker/buildx/cmd/buildx@v0.5.1
failed to run go list -find -json github.com/docker/buildx/cmd/buildx: exit status 1
go: github.com/moby/buildkit@v0.8.1-0.20201205083753-0af7b1b9c693 requires
	github.com/jaguilar/vt100@v0.0.0-20150826170717-2703a27b14ea (replaced by github.com/tonistiigi/vt100@v0.0.0-20190402012908-ad4c4a574305): missing go.sum entry; to add it:
	go mod download github.com/jaguilar/vt100

[jsternberg]

This should be possible at the moment. We don't have any excludes or replaces in the go.mod file at the moment.